New digital cellular communication systems, such as the Wideband Code Division Multiple Access (WCDMA) extension of the Global System for Mobile Communication (GSM) and Digital Cellular System (DCS) can utilize different operating modes for the transfer of digital information. For example, digital information can be transferred using two different duplex modes, Frequency Division Duplex (FDD) and Time Division Duplex (TDD), as are known in the art, and use different operating frequency bands. The GSM system operates in the 900, 1800 and 1900 MHz bands. In addition, a communication can share CDMA and Time Division Multiple Access (TDMA) aspects.
Multimode communication devices are designed to transmit and receive digital communications using operating systems chosen from a plurality of multiple access techniques including TDMA, CDMA, and GSM, and will combine some of these techniques and incorporate them into one device. The receiver portion of a dual mode communication device for example, is similar to those which are not dual mode but are adapted to receive a combination of signals in accordance with any of the systems above. For example, a device operating in a FDD mode can be transmitting in an uplink and receiving on a downlink on one operating system and receiving on a downlink on another operating system. In another example, a device can be required to occasionally operate in compressed mode, wherein the transmitter is turned off during certain reception periods.
In compressed mode, the gaps in reception are scheduled within the slot/frame structure so as to provide a transmission gap leaving an open time period for the device to perform interfrequency power measurement, acquisition of a control channel of another base station, and handover, for example. When in compressed mode, the information normally transmitted during a frame is compressed in time in order to maintain the amount of data transferred within a frame. However, the network system is penalized in capacity lost by the compressed event.
One method to alleviate data throughput problems associated with utilizing a compressed mode is to have a second receiver in the communication device. The use of a second receiver eliminates the requirement for the communication device to use a compressed mode in the downlink. However, it may still be a requirement for the device to use a compressed uplink mode. For example, a communication device can be transmitting on the uplink while receiving on the downlink and monitoring another downlink using the second receiver. Unfortunately, there are cases where the transmitter and receiver frequencies can crossmodulate causing intermodulation distortion. In particular, crossmodulation arises when the communication device transmitter leakage modulates an interferer in a receiver. Crossmodulation manifests itself in receiver stages with insufficient linearity such as mixers and amplifiers. The non-linearities create higher-order frequency intermodulation products which may fall in-band in a receiver causing interference. In particular, a receiver gain stage, can be a large source for intermodulation distortion due to its early placement in the receiver input path (i.e. the gain stage has the most exposure to transmitter leakage). A mixer also manifests undesired non-linear behavior and can allow crossmodulation. Crossmodulation problems are understood in duplex CDMA systems where it leads to high third-order intercept performance requirements and thus high current dissipation requirements. Therefore, it has often been a choice, while receiving a non-duplex signal in a multi-mode device, to use a compressed mode to allow non-transmission time for the device to accurately receive signals without distortion or interference caused by crossmodulation.
In practice, the typical receiver circuitry in a communication device comprises a preselector portion that functions to perform initial filtering, an amplifier portion for amplification of the desired signal within a bandwidth, and a mixer portion providing frequency conversion of the signal to an intermediate frequency (or direct conversion) for further processing by a backend portion of the receiver which performs digital signal processing on the signal. Controlling the amplification of the incoming signal power of a radio frequency receiver is essential to maintain adequate signal levels within successive stages of the receiver within the operating range of the front end and later stages of baseband circuitry and provide proper operation of the receiver. Out-of-band signals, such as signals out of the licensed spectrum band, and in particular, signals out of the channelized band can be interferers. This out-of-band signal power along with intermodulation products degrade receiver performance resulting in decreased signal-to-noise (or carrier-to-interference) ratio and degraded receiver sensitivity. This may occur when interfering signals are strong compared to the desired in-channel signal, i.e. blocking, or strong with respect to the usable linear region of the various stages within a receiver. This results in the desired in-channel signal being degraded due to undesired signals being translated (frequency shifted) into the desired band. Therefore, it is necessary to maintain the received signal power at various stages (RF, IF and baseband) within their respective usable linear operating signal power ranges.
Filtering is ineffective here since interference and intermodulation products may have been frequency translated to the desired channel, allowing them to pass through to the receiver backend. Compressed mode can alleviate some of these problems, however, it is undesirable to use compressed mode since system capacity is reduced and potentially data throughput reduced in the event that the device is resource limited. It is preferable to operate in an uncompressed mode, i.e. to simultaneously receive GSM and WCDMA while transmitting WCDMA. This mode is preferred by some service providers since it improves capacity. If compressed mode is used, it is very desirable to limit its use. Therefore, there is a need to reduce the effects of intermodulation distortion in an uncompressed mode.
Although the prior art has addressed crossmodulation in duplex CDMA receivers, some systems, such as a stand alone GSM receiver, do not have the performance to meet crossmodulation and high third-order intercept point requirements. Receiver design enhancements for the RF circuits are necessary to create a multimode handset used in uncompressed mode receiver architectures. For example, the low noise amplifier (LNA) for existing GSM receivers has a −5 dBm third-order intercept, whereas the low noise amplifier intercept point required during uncompressed reception is +11 dBm when considering the additional burden of crossmodulation and using a state of the art duplexer. Filtering out the transmitter power further within the duplexer or a diplexed filter is one solution which is currently beyond the available technology. Therefore, in order to meet the carrier-to-interference requirements, a low-noise amplifier (LNA) with exceptional linearity would be required. However, this adds to the cost of the device and would also lead to excessive current dissipation.
Reduction of distortion with concurrent decrease in linearity requirements can be achieved in multiple ways. One method is to use interference cancellation. However, interference cancellation techniques have not been viable to date due to implementation complexities. Another method is to use tunable RF circuits with increased selectivity. Again, tunable RF circuits/filters are not a viable technology to date. Also, a tunable filter may not help the crossmodulation problem, such as if a GSM reception is on one side of the receiver frequency band and a WCDMA reception/transmission is on the other side of the receiver frequency band.
Another technique to reduce the current drain penalty is to use linearity-on-demand systems, which dynamically adjust the linearity of a receiver based on transmitter power levels, received channel signal strength, and channel quality measurements (e.g. frame error rate or Ec/Io) as a metric to change the gain or current to the receiver channel gain stage. However, channel quality can be degraded by other things besides intermodulation. Therefore, the prior art would improperly supply corrections (requiring more current) even if intermodulation were not the cause of the present degraded signal condition. Moreover, the above techniques are limited to all CDMA systems where a CDMA quality metric is used and thus these methods do not address the GSM case. Furthermore, these techniques do not address the simultaneous reception scenario where the mobile device can switch to uplink compressed mode if needed to avoid the crossmodulation degradation in the non-duplex receiver.
Therefore, there is a need to improve reception in a multimode communication device, particularly with respect to uncompressed and GSM modes. It would also be of benefit to provide this improvement without the need for consistently maintaining high linearity stages and their associated current drain. It would also be an advantage if distortion reduction can be accommodated with a shared gain stage between the receiver channels. It would also be an advantage if a device can operate in compressed mode under certain scenarios to avoid crossmodulation but would be operated in uncompressed mode the rest of the time, and for normal traffic cases. It would also be advantageous to provide these improvements without significant additional hardware or cost in the communication device.